Semiconductor nanocrystal, and method of preparing the same

ABSTRACT

A nanocrystal including a core including a Group III element and a Group V element, and a monolayer shell on the surface of the core, the shell including a compound of the formula ZnSe x S (1-x) , wherein 0≤x≤1, and wherein an average mole ratio of Se:S in the monolayer shell ranges from about 2:1 to about 20:1.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/234,694, filed on Dec. 28, 2018, which is a continuation of U.S.patent application Ser. No. 14/039,788, filed on Sep. 27, 2013, andissued as U.S. Pat. No. 10,170,648, and claims priority to Korean PatentApplication No. 10-2012-0119887, filed on Oct. 26, 2012, and all thebenefits accruing to therefrom under 35 U.S.C. § 119, the contents ofwhich in their entirety are herein incorporated by reference.

BACKGROUND 1. Field

A semiconductor nanocrystal and a method of preparing the same aredisclosed.

2. Description of the Related Art

Semiconductor nanocrystals, which are also called quantum dots, havenano-sized particles with a crystalline structure and include hundredsto thousands of atoms.

The semiconductor nanocrystals are very small and thus have a largesurface area per unit volume, and also provide a quantum confinementeffect and the like. Accordingly, the semiconductor nanocrystals haveunique physicochemical properties that differ from the inherentcharacteristics of a corresponding bulk semiconductor material.

In particular, the optoelectronic properties of the semiconductornanocrystals may be tuned by controlling the size of the nanocrystals.Accordingly, new nanocrystals having improved properties and the usethereof in various applications are continuously sought.

SUMMARY

An embodiment provides a semiconductor nanocrystal having excellentlight emitting properties and stability.

Another embodiment provides a method of preparing the semiconductornanocrystal.

According to an embodiment, provided is a nanocrystal including a coreincluding a Group III element and a Group V element; and a monolayershell disposed on the surface of the core, the shell including acompound of the formula ZnSe_(x)S_((1-x)), wherein 0≤x≤1, and wherein anaverage mole ratio of Se:S in the monolayer shell ranges from about 2:1to about 20:1.

The core may be a binary element compound selected from GaN, GaP, GaAs,GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, and InSb, a ternary elementcompound selected from GaNP, GaNAs, GaNSb, GaPAs, GaPSb, AlNP, AlNAs,AlNSb, AlPAs, AlPSb, InNP, InNAs, InNSb, InPAs, InPSb, GaAlNP, AlGaN,AlGaP, AlGaAs, AlGaSb, InGaN, InGaP, InGaAs, InGaSb, AlInN, AlInP,AlInAs, and AlInSb, a quaternary element compound selected from GaAlNAs,GaAlNSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb,InAlNP, InAlNAs, InAlNSb, InAlPAs, and InAlPSb. A combination includingat least one of the foregoing may be used.

In an embodiment, the core may include InP.

The core may further include a Group II metal.

The Group II metal may be selected from Zn, Cd, Hg, Mg, and acombination thereof.

For example, when the core includes a Group II metal, the core may beInZnP.

In the nanocrystal, a mole ratio of Se:S in the monolayer shell rangesfrom about 3:1 to about 10:1.

The nanocrystal may further include at least one additional monolayer onthe monolayer shell.

The additional monolayers may include about two or more, for example,about 3 or more, or about 4 or more monolayers.

The about two or more additional monolayers may have a Se:Sconcentration ratio gradient among the additional monolayers.

For example, the gradient can be a decreasing gradient with a graduallyhigher concentration of S or a gradually lower concentration of Se fromthe additional monolayer closest to the core to the additional monolayerfarthest away from the core.

In an embodiment, the nanocrystal may include a ZnS layer as anoutermost layer.

Alternatively, the gradient can include an increasing gradient with agradually higher concentration of Se or a gradually lower concentrationof S from the additional monolayer closest to the core to an additionalmonolayer farther from the core, and a decreasing gradient with agradually higher concentration of S or a gradually lower concentrationof Se from the additional monolayer farther from the core to theadditional monolayer farthest away from the core such that theconcentration of Se may be lower than that of S.

In an embodiment, the nanocrystal may also include a ZnS layer as anoutermost layer.

The nanocrystal according to an embodiment may have a full width at halfmaximum of less than or equal to about 45 nm.

The nanocrystal may have a luminous efficiency, QY, of greater than orequal to about 70%, specifically greater than or equal to about 80%, ormore specifically about 90%.

The nanocrystal may have a diameter of greater than or equal to about 6nm, for example, greater than or equal to about 7 nm, or greater than orequal to about 8 nm.

The nanocrystal may have a light emitting region of about 500 to about750 nm.

According to another embodiment, a method of preparing a nanocrystalincluding a core including a Group III element and a Group V element,and a monolayer shell formed on a surface of the core, the methodincluding providing a nanocrystal core including a Group III element anda Group V element, and contacting the nanocrystal core with a precursorof Zn, Se and S to form a monolayer shell on a surface of thenanocrystal core and prepare the nanocrystal, wherein the precursor ofZn, Se, and S is present in an amount such that an average mole ratio ofSe:S in the monolayer shell ranges from about 2:1 to about 20:1.

In the method, the Zn, Se, and S precursors are present in a mole ratioranging from about 1:2 to about 60:1 to form the monolayer shell havinga Se:S mole ratio of from about 2:1 to about 20:1.

The core may further include a Group II metal.

In the method, at least one additional monolayer may be further formedon the ZnSeS monolayer shell.

The additional monolayer may be formed by introducing an additional Seand S precursor. In addition, the additional monolayer may be formed byintroducing additional Se, S, and Zn precursors.

The number of additional monolayers may be more than one.

The more than one additional monolayers may be formed by introducingadditional Se and S precursors with a mole ratio of the Se and Sprecursors such that Se and S may have a concentration ratio gradientamong the additional monolayers. In addition, the more than oneadditional monolayer may be formed by introducing additional Zn, Se, andS precursors in a mole ratio such that Se and S may have a concentrationratio gradient among the additional monolayers.

For example, the gradient can be a decreasing gradient with a graduallyhigher concentration of S or a gradually lower concentration of Se fromthe additional monolayer closest to the core to the additional monolayerfarthest away from the core.

Alternatively, the gradient gradually includes an increasing gradientwith a gradually higher concentration of Se or a gradually lowerconcentration of S from the additional monolayer closest to the core toan additional monolayer farther from the core, and a decreasing gradientwith a gradually higher concentration of S or a gradually lowerconcentration of Se from the additional monolayer farther from the coreto the additional monolayer farthest away from the core such that theconcentration of Se may finally be lower than that of S.

The method may further include formation of a ZnS layer as an outermostlayer.

The ZnS outermost layer may be formed by introducing an additional Sprecursor or additional S and Zn precursors.

According to another embodiment, a light emitting device including thenanocrystal according to the above embodiment is provided.

The light emitting device may be a display, a sensor, a photodetector, asolar cell, a hybrid composite, or a bio-labeling device.

In accordance with an embodiment, a nanocrystal including a core andhaving improved stability, luminescence efficiency, and full width athalf maximum (“FWHM”) is obtained.

The method of preparing the nanocrystal in accordance with theembodiments is highly reproducible.

BRIEF DESCRIPTION OF THE DRAWINGS

A description of the figures, which are meant to be exemplary and notlimiting, is provided in which:

FIGS. 1A and 1B are schematic views of an exemplary embodiment of ananocrystal including a ZnSeS monolayer shell formed on a Group III-Vcore, wherein

FIG. 1A schematically and exaggeratively shows that ZnSe isnon-uniformly bonded with ZnS to form a shell layer on the core, andFIG. 1B schematically shows that the ZnSe and ZnS uniformly form theshell layer on the core;

FIG. 2 is a schematic view of an exemplary embodiment of a nanocrystalincluding a shell having several ZnSeS monolayers in a concentrationgradient on a Group III-V core;

FIGS. 3A to 3C is a Transmission Electron Microscope (“TEM”) photographshowing the nanocrystals prepared according to Preparation Example 1,Comparative Example 3, and Example 4, wherein FIG. 3A is a TEMphotograph of the InZnP core according to Preparation Example 1, FIG. 3Bis a TEM photograph of the InZnP/ZnS nanocrystal according toComparative Example 3, and FIG. 3C is a TEM photograph of theInZnP/ZnSeS nanocrystal according to Example 4;

FIG. 4 is a graph of absorbance (arbitrary units, a.u.) versuswavelength (nanometers, nm) showing absorbance of the InZnP nanocrystalcore according to Preparation Example 1, the InZnP/ZnS nanocrystalaccording to Comparative Example 3, and the InZnP/ZnSeS nanocrystalaccording to Example 4;

FIG. 5 is a graph of photoluminescence (“PL”) intensity (arbitraryunits, a.u.) versus wavelength (nanometers, nm) showing light emittingproperties of the InZnP nanocrystal core according to PreparationExample 1, the InZnP/ZnS nanocrystal according to Comparative Example 3,and the InZnP/ZnSeS nanocrystal according to Example 4;

FIGS. 6A and 6B provides in FIG. 6A a graph schematically showing abandgap difference among InP, ZnSe, and ZnS, and in FIG. 6B a tableshowing each lattice constant of the InP, ZnSe, and ZnS;

FIG. 7 is a graph showing absorbance (arbitrary units, a.u.) versuswavelength (nanometers, nm) of the nanocrystals including a ZnSeS or ZnSmonolayer shell formed on an InP core, the nanocrystals furtherincluding a ZnSeS monolayer shell on the aforementioned nanocrystalsaccording to Examples 1 to 3 and Comparative Examples 1 and 2;

FIG. 8 is a graph of photoluminescence intensity (arbitrary units, a.u.)versus wavelength (nanometers, nm) showing light emitting properties ofthe nanocrystals including a ZnSeS or ZnS monolayer shell formed on anInP core, the nanocrystals further including a ZnSeS monolayer shell onthe aforementioned nanocrystal according to Examples 1 to 3 andComparative Examples 1 and 2;

FIG. 9 is a graph of absorbance (arbitrary units, a.u.) versus ofwavelength (nanometers, nm) showing absorbance of the nanocrystalaccording to Example 7;

FIG. 10 is a graph of photoluminescence intensity (arbitrary units,a.u.) versus wavelength (nanometers, nm) showing light emittingproperties of the nanocrystal according to Example 7; and

FIG. 11 is a TEM photograph showing the nanocrystal according to Example6.

DETAILED DESCRIPTION

This disclosure will be more fully described hereinafter in thefollowing detailed description, in which some but not all embodiments ofthis disclosure are described.

This disclosure may be embodied in many different forms and is not beconstrued as limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will fully convey thescope of the disclosure to those skilled in the art.

In the drawings, the thickness of layers, films, panels, regions, etc.are exaggerated for clarity.

It will be understood that when an element such as a layer, film,region, or substrate is referred to as being “on” another element, itcan be directly on the other element or an intervening element may alsobe present. In contrast, when an element is referred to as being“directly on” another element, there is no intervening element present.

As used herein, the term “combination thereof” refers to a mixture, astacked structure, a composite, an alloy, a blend, a reaction product,or the like.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. The term“or” means “and/or.” As used herein, the term “and/or” includes any andall combinations of one or to more of the associated listed items.Expressions such as “at least one of,” when preceding a list ofelements, modify the entire list of elements and do not modify theindividual elements of the list.

It will be further understood that the terms “comprises” and/or“comprising,” or “includes” and/or “including” when used in thisspecification, specify the presence of stated features, regions,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,regions, integers, steps, operations, elements, components, and/orgroups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this general inventive conceptbelongs. It will be further understood that terms, such as those definedin commonly used dictionaries, should be interpreted as having a meaningthat is consistent with their meaning in the context of the relevant artand the present disclosure, and will not be interpreted in an idealizedor overly formal sense unless expressly so defined herein.

Exemplary embodiments are described herein with reference to crosssection illustrations that are schematic illustrations of idealizedembodiments. As such, variations from the shapes of the illustrations asa result, for example, of manufacturing techniques and/or tolerances,are to be expected. Thus, embodiments described herein should not beconstrued as limited to the particular shapes of regions as illustratedherein but are to include deviations in shapes that result, for example,from manufacturing. For example, a region illustrated or described asflat may, typically, have rough and/or nonlinear features. Moreover,sharp angles that are illustrated may be rounded. Thus, the regionsillustrated in the figures are schematic in nature and their shapes arenot intended to illustrate the precise shape of a region and are notintended to limit the scope of the present claims.

As used herein, the term “Group III-V core” refers to a core comprisinga Group III element and a Group V element. A Group III-V core may be abinary element compound, a ternary element compound, a quaternaryelement compound or a combination thereof.

“Group” refers to a Group of the Periodic Table of the Elements.

According to an embodiment, a nanocrystal includes

a core 10, e.g., a Group III-V core; and

a monolayer shell 11, e.g., a ZnSeS monolayer (1 monolayer) shell,disposed on the surface of the core, the shell comprising a compound ofthe formula ZnSe_(x)S_((1-x)), wherein 0≤x≤1, and

wherein an average mole ratio of Se:S in the monolayer shell ranges fromabout 2:1 to about 20:1.

In an embodiment, the mole ratio of Se:S in the ZnSeS monolayer on thesurface of the core may range from about 3:1 to about 10:1, from about3:1 to about 9:1, from about 3:1 to about 8:1, or from about 4:1 toabout 10:1.

The Group III-V core may be a binary element compound selected from GaN,GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, and InSb, aternary element compound selected from GaNP, GaNAs, GaNSb, GaPAs, GaPSb,AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InNP, InNAs, InNSb, InPAs, InPSb,GaAlNP, AlGaN, AlGaP, AlGaAs, AlGaSb, InGaN, InGaP, InGaAs, InGaSb,AlInN, AlInP, AlInAs, and AlInSb, a quaternary element compound selectedfrom GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs, GaInNSb,GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs, and InAlPSb, and acombination thereof.

In an embodiment, the Group III-V core may be an InP core.

The Group III-V core may include a Group II metal besides a Group III-Vmetal.

The Group II metal may be selected from Zn, Cd, Hg, Mg, and acombination thereof.

For example, when the Group III-V core includes a Group II metal, thecore may be an InZnP core.

The nanocrystal may further include at least one ZnSeS monolayer on theZnSeS monolayer shell.

The ZnSeS monolayer may include about 2 or more, for example, about 3 ormore, or about 4 or more monolayers.

Herein, a monolayer is formed in which each ZnS or ZnSe molecule isbonded on the surface of the Group III-V core. In other words, noadditional layer is formed by bonding additional ZnS or ZnSe moleculeson the layer formed by consecutively bonding ZnS or ZnSe molecules onthe core.

As illustrated in FIG. 1A, in an embodiment, ZnSe is non-uniformlybonded with ZnS, e.g., to provide a ZnSe portion 12 and a ZnS portion13, to form a shell layer on the core. In another embodiment, ZnSe andZnS uniformly form the shell layer on the core, as illustrated in FIG.1B.

When at least two additional ZnSeS monolayers are formed, as shown inFIG. 2, Se and S may have a concentration ratio gradient among the atleast two additional ZnSeS monolayers.

The additional ZnSeS monolayers may gradually include S at a higherconcentration than Se when the monolayers are farther away from thecore. Accordingly, the nanocrystal may have a ZnS layer as an outermostlayer 21.

Alternatively, the additional ZnSeS monolayers may gradually include Sein a higher concentration than S when the monolayers are farther awayfrom the core first, but then include Se in a gradually lowerconcentration and the S in a gradually higher concentration afterforming a predetermined number of ZnSeS monolayers.

For example, the Se is gradually included in a higher concentrationratio than the S until 1 to 2 additional monolayers are formed on theZnSeS monolayer shell, but gradually in a lower concentration ratio fromthe third additional layer, while S is gradually included in a higherconcentration forming a concentration gradient.

Recently, quantum dot (QD) nanocrystals having an energy band gap thatis adjusted depending on size and composition and having excellent lightemitting properties such as high color purity have drawn attention as amaterial that is variously applied in display, semiconductor, energy,and bio fields.

In particular, the quantum dot (QD) nanocrystals have been moreapplicable due to development of new synthesis technology foraccomplishing high quality by adjusting size and shape, structure,uniformity, and the like in a colloid in a simple wet method.

In addition, the QD nanocrystals including conventional Cd containingnanocrystals have been reported to have excellent characteristics.However, since the Cd nanocrystals cause an environmental problem, anattempt to develop a material including no Cd has been made. As for thenew material, a Group III-V nanocrystal has been researched. Theprecursors of Group III-V nanocrystals, however, are more sensitive tooxidation than those for CdSe-based QDs during synthesis and aredeteriorated in activity, thus it is difficult to control synthesisusing precursors of Group III-V nanocrystals. As for the Group III-Vnanocrystal, InP/ZnS has been mostly researched, but often hasefficiency of less than or equal to about 60%, a full width at halfmaximum (FWHM) of greater than or equal to about 40 nm, and a particlesize ranging from about 2 to about 5 nm, and thus has lower lightemitting properties than the conventional CdSe-based QD (referring toNano Lett. 2012, 12, 3986., ACS nano, 2011, 5 12, 9392., Appl. Phys.Lett. 2012, 96, 073102., J. Phys. Chem. Lett. 2012, 3, 214.).

Accordingly, there is a need for development of a QD nanocrystal havinghigher quantum efficiency and higher color purity that can be applied toa photoelectron device, a sensor, or the like.

According to an embodiment, a nanocrystal has remarkably improved toluminous efficiency (QY) and full width at half maximum (FWHM). Thenanocrystal also stably grows to have a QD size of greater than or equalto about 6 nm by forming a ZnSeS monolayer including Se:S in a moleratio ranging from about 2:1 to about 20:1 on the surface of a GroupIII-V core, and then additional forming ZnSeS monolayers having aconcentration ratio gradient thereon. For example, the additionalmonolayer may include S in a gradually higher concentration than the Sewhen the additional monolayer is farther away from the core.

In particular, the nanocrystal according to an embodiment may have afull width at half maximum of less than or equal to about 45 nm andluminous efficiency of greater than or equal to about 70%, specifically,greater than or equal to about 80%, or more specifically about 90%.

The nanocrystal may have a diameter of greater than or equal to 6 nm,for example, greater than or equal to about 7 nm or greater than orequal to about 8 nm.

The nanocrystal may have a light emitting region of about 500 to about750 nm.

Seoul National University published an article (Chem. Mater. 2011, 23,4459-4463) disclosing an InP/ZnSeS nanocrystal including Se:S in a moleratio of 1:1.2 in a contact layer with a core but in a mole ratio of 1:7farther from the core as a shell grows bigger.

However, a nanocrystal including a Group III-V core and a ZnSeSmonolayer formed on the surface of the core and including Se:S in a moleratio ranging from about 2:1 to about 20:1, for example, about 3:1 toabout 10:1 according to an embodiment of the present disclosure may havemuch better light emitting properties than the one disclosed in theChem. Mater. by forming additional ZnSeS monolayers thereon.

Specifically, the nanocrystal may stably grow by forming additionalZnSeS monolayers on the ZnSeS monolayer shell formed on the surface ofthe core and having Se:S in a predetermined concentration ratio. Herein,the additional ZnSeS monolayers include Se and S with a concentrationratio gradient farther from the core. Accordingly, the nanocrystal hasmuch better light emitting properties than QY<50% and full width at halfmaximum (FWHM) of 70 nm published in Chem. Mater.

The concentration gradient between Se and S among the additional ZnSeSmonolayers does not necessarily mean that S is in a higher concentrationand Se is in a lower concentration when the monolayer is farther awayfrom the core. The additional ZnSeS monolayers may have the Se in ahigher concentration up to a predetermined size and then the S in ahigher concentration.

In other words, the nanocrystal including a Group III-V core and a ZnSeSmonolayer formed on the surface of the core and including Se:S in aratio ranging from about 2:1 to about 20:1 according to an embodiment ofthe present disclosure may maintain a predetermined ratio of Se:S in themonolayer, which may further stabilize additionally-formed ZnSeSmonolayers thereon and secure excellent light emitting properties.

According to an embodiment, the nanocrystal may be prepared by forming aGroup III-V nanocrystal core and then a ZnSeS monolayer including Se:Sin a ratio ranging from about 2:1 to about 20:1 on the surface of thecore.

Another embodiment of the present disclosure provides a method ofpreparing the nanocrystal including a Group III-V core and a ZnSeS shellformed on a surface of a ZnSeS shell, including

preparing a Group III-V nanocrystal core, and

forming a ZnSeS monolayer shell from precursors of Zn, Se and S on thesurface of the nanocrystal core,

wherein the Zn, Se, and S precursors are present in such amounts thatthe ZnSeS monolayer shell includes Se:S in a mole ratio ranging fromabout 2:1 to about 20:1.

The ZnSeS monolayer including Se:S in a mole ratio ranging from about2:1 to about 20:1 on the Group III-V core is formed by introducing Seand S precursors in a ratio ranging from about 1:2 to about 60:1.

Since the S precursor has less reactivity than the Se precursor, theZnSeS monolayer on the surface of the Group III-V core may maintain atleast a mole ratio of about 2:1 between Se:S despite introducing the Seand S precursors in a ratio of 1:2.

Accordingly, in order to prepare the nanocrystal according to theembodiment of the present disclosure, the Se and S precursors forpreparing the ZnSeS monolayer on the surface of a Group III-V core maybe present in a mole ratio ranging from about 1:2 to about 60:1.

The method may further include, for example, additional formation of atleast two ZnSeS monolayers on the Group III-V core and the ZnSeSmonolayer shell.

The additional formation of at least two ZnSeS monolayers may includeintroduction of additional Se and S precursors. The Se and S precursorsmay be introduced by adjusting their mole ratio to form a concentrationgradient between Se and S among the additional monolayers. In addition,the additional formation of at least two ZnSeS monolayers may includeintroduction of additional Zn, Se, and S precursors by adjusting theirmole ratio to form a concentration ratio gradient between Se and S amongthe additional monolayers.

The gradient gradually includes S in a higher concentration than Sefarther from the core. Accordingly, the method may further includeformation of a ZnS layer as an outermost layer.

Alternatively, the gradient gradually includes Se in a higherconcentration than S father from the core up to a predetermined layer,but then Se in a gradually lower concentration. Accordingly, the methodmay further include formation of a ZnS layer as an outermost layer.

The ZnS outermost layer may be formed by sequentially introducing an Sprecursor, or S precursor along with Zn precursor on the nanocrystalincluding the Group III-V core and the ZnSeS monolayer formed on thesurface of the core.

According to an embodiment, the method of preparing a nanocrystal may beperformed by sequentially forming at least two ZnSeS monolayers on thesurface of a Group III-V core after forming the Group III-V core in onereactor. Herein, the additional ZnSeS monolayers are formed bysequentially introducing Se, S, and/or Zn precursors into the samereactor.

However, the method does not need to be sequentially performed in onereactor, but may be performed by separating it from one reactor afterpreparing a nanocrystal with a predetermined size in the reactor andthen forming an additional monolayer thereon in another reactor ifnecessary. In other words, each additional monolayer is formed in eachdifferent reactor by separating a nanocrystal from one reactor afterforming it in the former reactor. This additional layer coating processmay be repeated several times.

According to the embodiment of the present disclosure, the process mayhave high reproducibility. As shown in Examples 4 and 5 describedherein, a nanocrystal formed by sequentially forming ZnSeS monolayers onthe surface of a Group III-V core and another nanocrystal formed byseparating it after growing to a predetermined size and formingadditional ZnS layers may have excellent light emitting properties andstable growth.

On the other hand, a ZnS shell may be easily formed on a Group III-Vcore, specifically, an InP core or an InZnP core with high efficiencyaccording to an embodiment of the present disclosure as shown in Example5. The Group II-VI ZnS shell may be formed on the Group III-V InP core.However, it is hard to stably form a nanocrystal having the core-shellstructure. The reason is that InP used as the core and ZnS used as theshell have a large lattice constant difference, as shown in FIGS. 6B.However, when ZnSeS is formed by including Se in a higher ratio towardthe core and S in a higher ratio toward the outer layer, a uniform andthick shell is formed by decreasing the lattice constant difference. Asa result, the nanocrystal has a decreased full width at half maximum andimproved luminous efficiency and stability.

These results are provided in FIGS. 3 to 5.

FIGS. 3A to 3C are a TEM photographs showing the nanocrystals preparedaccording to Preparation Example 1, Comparative Example 3, and Example4, wherein FIG. 3A is a TEM photograph showing the InZnP core preparedin Preparation Example 1, FIG. 3B is a TEM photograph showing theInZnP/ZnS nanocrystal prepared in Comparative Example 3, and FIG. 3C isa TEM photograph showing the InZnP/ZnSeS nanocrystal prepared in Example4.

FIG. 4 is a graph showing absorbance of the InZnP nanocrystal coreaccording to Preparation Example 1, the InZnP/ZnS nanocrystal accordingto Comparative Example 3, and the InZnP/ZnSeS nanocrystal according toExample 4.

FIG. 5 is a graph showing photoluminescence intensity of the InZnPnanocrystal core according to Preparation Example 1, the InZnP/ZnSnanocrystal according to Comparative Example 3, and the InZnP/ZnSeSnanocrystal according to Example 4.

As shown in FIGS. 3 to 5, the InZnP/ZnSeS nanocrystal (FIG. 3C)according to Example 4 has a much bigger particle size than an InZnP/ZnSnanocrystal (FIG. 3B).

In addition, the InZnP/ZnSeS nanocrystal according to Example 4 hasexcellent luminous efficiency and decreased full width at half maximumcompared with the InZnP/ZnS nanocrystal according to Comparative Example3 based on FIGS. 4 and 5.

FIGS. 7 and 8 are graphs showing absorbance and light emittingproperties of the nanocrystal including a ZnSeS or ZnS monolayer on anInP core and the nanocrystal further including a ZnSeS monolayer shellon the aforementioned nanocrystal according to Examples 1 to 3 andComparative Examples 1 and 2.

Referring to FIG. 8, the nanocrystals including Se:S in the monolayerwithin the ratio according to the present disclosure and thenanocrystals including an additional ZnSeS monolayer on the monolayeraccording to Examples 1 to 3 have excellent light emitting propertiescompared with the nanocrystals including Se:S that falls outside of theratio of the present disclosure (Comparative Examples 1 and 2). FIGS. 9and 10 are graphs showing absorbance and PL light emitting properties ofthe InP/ZnSeS/ZnS nanocrystal including an InP core, a shell formed of aplurality of ZnSeS monolayers, and a ZnS layer as an outermost layeraccording to an exemplary embodiment of the present disclosure.

Based on these graphs, the nanocrystal having a plurality of ZnSeSmonolayers as a shell on the InP core and a ZnS shell layer as anoutermost layer according to the embodiment of the present disclosurehas excellent light emitting properties compared with a nanocrystalhaving a ZnS shell layer directly formed on an InP core like ComparativeExample 3.

Without being bound to a specific theory, the nanocrystal according tothe embodiment of the present disclosure includes a ZnSeS intermediateshell layer including a predetermined ratio between Se:S to decrease alattice constant difference between the InP core and the ZnS shell asschematically shown in FIG. 6, and thus may more stably grow.

According to an embodiment, the nanocrystal may be easily formed in acolloid wet process that is well known for manufacturing a nanocrystal.

In other words, the colloid wet process may be used to form a GroupIII-V core and a ZnSeS monolayer on the surface of the core, except thatit includes Se and S within the ratio according to the embodiment of thepresent disclosure.

In addition, when additional ZnSeS monolayers are formed on the surfaceof the nanocrystal, the colloid wet process may be used to form theadditional ZnSeS monolayers except that the ratio of Se:S is adjusted inthe additional ZnSeS monolayers.

The colloid wet process for forming a nanocrystal is well known to thosewho have common knowledge in a related art and will not be illustratedin detail.

According to another embodiment, a light emitting device including thenanocrystal according to the above embodiment is provided.

The light emitting device may be a display, a sensor, a photodetector, asolar cell, a hybrid composite, a bio-labeling device, and the like.

Hereinafter, the present disclosure is illustrated in more detail withreference to examples. However, they are exemplary embodiments of thepresent disclosure, and the present disclosure is not limited thereto.

EXAMPLES Preparation Example 1: Preparation of InZnP Core

0.2 mmol (0.058 g) of indium acetate, 0.125 mmol (0.0183 g) of zincacetate, 0.8 mmol (0.204 g) of palmitic acid, and 10 mL of 1-octadeceneare added to a flask and vacuum-treated at 120° C. for 1 hour. Themixture is heated up to 280° C. after nitrogen (N₂) is introduced intothe flask. The reaction temperature is set at 280° C. for stabilization,and a mixed solution of 0.15 mmol (43 μL) oftris(trimethylsilyl)phosphine and 1 mL of trioctyl phosphine (“TOP”) arerapidly added thereto. The resulting mixture is reacted for 10 minutes.The reactant is rapidly cooled down, centrifuged with acetone, and thendispersed in toluene. The obtained nanocrystal (QD) has a UV firstabsorption maximum ranging from 440 to 460 nm.

Preparation Example 2: Preparation of InP Core

0.2 mmol (0.058 g) of indium acetate, 0.6 mmol (0.154 g) of palmiticacid, and 10 mL of 1-octadecene are added to a flask and vacuum-treatedat 120° C. for one hour. The mixture is heat-treated up to 280° C. afterintroducing nitrogen (N₂) into the flask. When the reactant isstabilized by setting the reaction temperature at 280° C., a mixedsolution of 0.1 mmol (29 μL) of tris(trimethylsilyl)phosphine and 0.5 mLof TOP is rapidly added thereto. The resulting mixture is reacted for 40minutes. In addition, 0.2 mmol (0.058 g) of indium acetate, 0.6 mmol(0.154 g) of palmitic acid, and 4 mL of 1-octadecene are added toanother flask, vacuum-treated at 120° C. for 1 hour, and cooled down to50° C. After introducing nitrogen (N₂) into the flask, a mixed solutionof 0.1 mmol (29 μL) of tris(trimethylsilyl)phosphine and 0.5 mL of TOPis added to the first InP mixed solution in a dropwise fashion for 15minutes. The resulting mixture is further reacted for 10 minutes.

The obtained InP nanocrystal (QD) has a UV first absorption maximumranging from 565 to 575 nm.

Examples 1 to 3 and Comparative Examples 1 and 2: Preparation of ananocrystal including an InZnP core and a ZnSeS monolayer shell,preparation of a nanocrystal including an InZnP core and a ZnSeSmulti-layered shell, and light-emitting properties of the nanocrystals

Nanocrystals respectively including a ZnSeS monolayer and multi-layeredshells on the surface of the nanocrystal core according to PreparationExample 1 are formed.

Specifically, 0.3 mmol (0.056 g) of zinc acetate, 0.6 mmol (0.189 g) ofoleic acid, and 10 mL of trioctylamine are added to a flask andvacuum-treated at 120° C. for 10 minutes. The mixture is heated up to220° C. after introducing nitrogen (N₂) into the flask. Next, the InZnPcore according to Preparation Example 1 is added to the reactant within10 seconds. Then, Se/TOP is slowly added thereto to have a compositionin the following Table 1 related to the zinc acetate. The resultingmixture is heated up to 280° C. Then, S/TOP is added thereto to have acomposition in the following Table 1. The resulting mixture is heated upto 320° C. and reacted for 20 minutes.

After the reaction, the resulting product is cooled down, obtainingnanocrystal. The nanocrystal is centrifuged with ethanol and dispersedin toluene.

The nanocrystal is measured regarding PL characteristic having a ZnSeSmonolayer shell on an InZnP core. The result is provided in Table 1.

In addition, nanocrystals having a ZnSeS monolayer on an InZnP corehaving a ratio of Zn, Se, and S provided in the following Table 1 insidea ZnSeS monolayer shell are respectively formed in the same method asaforementioned. Then, additional ZnSeS layers are consecutively formedto form a ZnSeS multi-layered shell on the InZnP core.

In other words, a mixed solution of 0.01 mmol of Se/TOP and 0.05 mmol ofS/TOP is slowly injected into a reactor including each nanocrystalhaving a monolayer shell having a composition of the following Table 1,and the mixture is reacted for 20 minutes again. Then, a mixed solutionof Se and S is prepared in a different ratio, injected into the reactor,and reacted for 20 minutes. Various mixed solutions of 0.01 mmol ofSe/TOP+0.08 mmol of S/TOP, 0.01 mmol of Se/TOP+0.12 mmol of S/TOP, and0.01 mmol of Se/TOP+0.15 mmol of S/TOP are sequentially used.

When the reaction is complete, the reactor is cooled down, and theobtained nanocrystal is centrifuged with ethanol and dispersed intoluene.

The nanocrystal is measured regarding the PL characteristic of having aZnSeS multi-layered shell on an InZnP core. The result is provided inTable 1.

TABLE 1 PL characteristic of a PL characteristic of a monolayerstructure: multi-layer structure: Se:S Composition light emitting lightemitting ratio of in a ratio of wavelength (full width wavelength (fullwidth ZnSeS reactants at half maximum) at half maximum) monolayer(Zn:Se:S) luminous efficiency luminous efficiency Comparative 1:01:1.83:0 540 (48) 15% 544 (44) 39% Example 1 Example 1 12:1  1:0.1:0.033534 (41) 41% 536 (38) 70% Example 2 9:1 1:0.067:0.033 535 (42) 31% 533(39) 78% Example 3 5:1 1:0.067:0.067 535 (43) 47% 535 (39) 70%Comparative 1:1 1:0.033:0.1 527 (49) 55% 531 (46) 40% Example 2

In addition, the nanocrystals respectively having a monolayer shell anda multi-layered shell are measured regarding UV absorbance and PL lightemitting properties. The results are respectively provided in FIGS. 7and 8.

In FIGS. 7 and 8, each dotted line shows the UV absorbance or PL lightemitting properties of the nanocrystal having a monolayer shell, whileeach solid line shows the UV absorbance and PL light emitting propertiesof the nanocrystal having a multi-layered shell.

As shown in Table 1 and FIG. 8, when the Se and S in the first monolayercontacting the nanocrystal core has a ratio within the range accordingto an embodiment of the present disclosure, the nanocrystal havingadditional ZnSeS monolayers has improved light emitting properties, andparticularly decreased full width at half maximum (FWHM).

Example 4: Preparation of Nanocrystal InZnP/ZnSeS Including InZnP Coreand ZnSeS Multi-Layered Shell

0.3 mmol (0.056 g) of zinc acetate, 0.6 mmol (0.189 g) of oleic acid,and 10 mL of trioctylamine are added to a flask and vacuum-treated at120° C. for 10 minutes. The vacuum-treated mixture is heated up to 220°C. after introducing nitrogen (N₂) into the flask. Then, the InZnP coreaccording to Preparation Example 1 is added thereto within 10 seconds,and 0.02 mmol of Se/TOP is slowly injected thereinto. The resultingmixture is heated up to 280° C. Next, 0.01 mmol of S/TOP is addedthereto. The obtained mixture is heated up to 320° C. and reacted for 20minutes. Consecutively, a mixed solution of 0.01 mmol of Se/TOP and 0.05mmol of S/TOP is slowly injected thereto and reacted for 20 minutesagain. Then, the same reaction is repeated for 20 minutes by switchingthe ratio between Se and S. The Se and S are mixed in a ratio of 0.01mmol of Se/TOP+0.08 mmol of S/TOP, 0.01 mmol of Se/TOP+0.12 mmol ofS/TOP, 0.01 mmol of Se/TOP+0.15 mmol of S/TOP, and 0.19 mmol of S/TOP.These mixed solutions are sequentially used.

When the reaction is complete, the reactor is cooled down, and theobtained nanocrystal is centrifuged with ethanol and dispersed intoluene.

The obtained nanocrystal (QD) has a UV first absorption maximum rangingfrom 510 to 515 nm, a PL emission peak ranging from 535 to 545 nm, aFWHM ranging from 38 to 44 nm, and QY ranging from 70 to 80%.

Example 5: Preparation of Nanocrystal InZnP/ZnSeS/ZnS Including InZnPCore, ZnSeS Multi-Layered Shell, and ZnS Outermost Layer

0.3 mmol (0.056 g) of zinc acetate, 0.3 mmol (0.0947 g) of oleic acid,and 10 mL of trioctylamine are added to a flask and vacuum-treated at120° C. for 10 minutes. The vacuum-treated mixture is heated up to 220°C. after introducing nitrogen (N₂) into the flask. Then, the nanocrystalInZnP/ZnSeS toluene solution according to Example 4 is added to theheated mixture within 10 seconds, and 6 mmol of S/TOP is slowly injectedthereinto. The resulting mixture is heated up to 280° C. and reacted for2 hours.

The obtained nanocrystal (QD) has a UV first absorption maximum rangingfrom 510 to 515 nm, a PL emission peak ranging from 535 to 545 nm, aFWHM ranging from 38 to 44 nm, and QY ranging from 70 to 90%.

Comparative Example 3: Preparation of Nanocrystal InZnP/ZnS Including anInZnP Core and a ZnS Layer

0.3 mmol (0.056 g) of zinc acetate, 0.6 mmol (0.0947 g) of oleic acid,and 10 mL of trioctylamine are added to a flask and vacuum-treated at120° C. for 10 minutes. The vacuum-treated mixture is heated up to 220°C. after introducing nitrogen (N₂) into the flask. Herein, the InZnPcore according to Preparation Example 1 is added thereto within 10seconds, and then 6 mmol of S/TOP is slowly injected thereinto. Theresulting mixture is heated up to 300° C. and reacted for 1 hour.

When the reaction is complete, the reactor is cooled down, and theobtained nanocrystal is centrifuged with ethanol and dispersed intoluene.

This nanocrystal includes a ZnS layer on an InZnP core.

The InZnP/ZnS nanocrystal including no ZnSeS intermediate shell layeraccording to Comparative Example 3, the InZnP core according toPreparation Example 1, and the InZnP/ZnSeS nanocrystal according toExample 4 are measured regarding UV absorbance and light emittingproperties. The results are respectively provided in FIGS. 4 and 5.

As shown in FIGS. 4 and 5, the nanocrystal including a monolayer havinga Se:S ratio according to the embodiment of the present disclosure hashigher absorbance than the nanocrystal including no monolayer accordingto Comparative Example 3, and thus has excellent QY luminous efficiencyand a decreased full width at half maximum.

The InP/ZnS nanocrystal according to Comparative Example 3 has too largean energy band gap between the InP core and the ZnS shell to be formed.Thus, the nanocrystal does not have excellent light emitting properties,as schematically shown in FIG. 6A. On the other hand, the nanocrystalincluding a ZnSeS shell layer according to Example 4 has remarkablyimproved UV absorbance and PL light emitting properties.

Example 6: Preparation of InZnP/ZnSeS/ZnS Nanocrystal Having a DifferentConcentration Ratio Gradient

0.3 mmol (0.056 g) of zinc acetate, 0.6 mmol (0.189 g) of oleic acid,and 10 mL of trioctylamine are added to a flask and then vacuum-treatedat 120° C. for 10 minutes. The vacuum-treated mixture is heated up to220° C. after introducing nitrogen (N₂) into the flask. Next, the InZnPcore according to Preparation Example 1 is added to the heated mixturewithin 10 seconds, and 0.02 mmol of Se/TOP is slowly injected thereinto.The resulting mixture is heated up to 280° C. Then, 0.02 mmol of S/TOPis added thereto, and the obtained mixture is heated up to 320° C. andreacted for 20 minutes. In addition, a mixed solution of 0.02 mmol ofSe/TOP+0.04 mmol of S/TOP is slowly added thereto and reacted for 20minutes. This same step was repeated by switching a mixing ratio of Seand S. The mixed solution of Se and S may be prepared by mixing 0.01mmol of Se/TOP+0.05 mmol of S/TOP, 0.005 mmol of Se/TOP+0.1 mmol ofS/TOP, 0.005 mmol of Se/TOP+0.2 mmol of S/TOP, and 0.2 mmol of a S/TOPsolution and sequentially using them.

When the reaction is complete, the reactor is cooled down, and theobtained nanocrystal is centrifuged with ethanol and dispersed intoluene.

Herein, the nanocrystal has a UV first absorption maximum ranging from510 to 515 nm, a PL emission peak ranging from 535 to 545 nm, a FWHMranging from 38 to 44 nm, and QY ranging from 70 to 80%. In addition,the TEM photograph of the nanocrystal is provided in FIG. 11. As shownin FIG. 11, the nanocrystal according to the present exemplaryembodiment has a relatively uniform average size of 6.05 nm.

According to the present exemplary embodiment, the Se/S ratio is 3.1 inthe first ZnSeS monolayer contacting the core and 5.0 in the secondZnSeS monolayer, that is, it becomes higher at first, and then becomesgradually lower, and finally becomes lowest down to 0.59 in theoutermost layer when the nanocrystal has the biggest diameter of 6 nm.

Based on the light emitting properties and full width at half maximumresults, when Se and S in the first monolayer contacting the core have aratio within the range according to the embodiment of the presentdisclosure, the Se and S do not necessarily maintain a specificconcentration gradient in the additional ZnSeS monolayers. In otherwords, the concentration gradient can have the S:Se ratio graduallyhigher from the core to the outer layer, as shown in Example 5.Alternatively, as shown in Example 6, Se:S ratio becomes graduallyhigher up to one to two layers from the core and becomes gradually lowerfarther away from the core. There seems no significant difference in thelight emitting properties between these two nanocrystals. As a result,the ratio between Se and S in the first monolayer contacting the core ofa nanocrystal seems to be important.

Example 7: Preparation of InP/ZnSeS/ZnS Nanocrystal

0.3 mmol (0.056 g) of zinc acetate, 0.6 mmol (0.189 g) of oleic acid,and 10 mL of trioctylamine are added to a flask and vacuum-treated at120° C. for 10 minutes. The vacuum-treated mixture is heated up to 180°C. when introducing nitrogen (N₂) in the flask. Next, the InP coreaccording to Preparation Example 2 and 0.03 mmol of Se/TOP are addedthereto within 10 seconds. The resulting mixture is heated up to 280° C.Then, 0.005 mmol of S/TOP is added to the heated mixture. The obtainedmixture is then heated up to 320° C. and reacted for 20 minutes. Next, amixed solution of 0.02 mmol of Se/TOP+0.01 mmol of S/TOP is slowlyinjected thereinto. The resulting mixture is reacted for 20 minutes.This same step is repeated by switching a mixing ratio of Se and S. Themixed solution is prepared by respectively mixing 0.01 mmol ofSe/TOP+0.05 mmol of S/TOP, 0.005 mmol of Se/TOP+0.15 mmol of S/TOP,0.005 mmol of Se/TOP+0.2 mmol of S/TOP, and 0.2 mmol of a S/TOPsolution, which are sequentially used.

When the reaction is complete, the reactor is cooled down, and theInP/ZnSeS nanocrystal is centrifuged with ethanol and dispersed intotoluene. In addition, 0.3 mmol (0.056 g) of zinc acetate, 0.3 mmol(0.0947 g) of oleic acid, and 10 mL of trioctylamine are added to aflask and vacuum-treated at 120° C. for 10 minutes. The vacuum-treatedmixture is heated up to 220° C. after substituting nitrogen (N₂) intothe flask. Herein, the InP/ZnSeS nanocrystal toluene solution is addedto the flask within 10 seconds, and 6 mmol of S/TOP is slowly injectedinto. The resulting mixture is heated up to 280° C. and reacted for 2hours.

The obtained nanocrystal (QD) has a UV first absorption maximum rangingfrom 585 to 595 nm, a PL light emitting peak ranging from 615 to 625 nm,and a FWHM ranging from 40 to 45 nm. In addition, the nanocrystal (QD)has luminous efficiency (QY) of 70%. The results are provided in FIGS. 9and 10.

While this disclosure has been described in connection with what is topresently considered to be practical exemplary embodiments, it is to beunderstood that the disclosure is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A nanocrystal comprising: a core comprising aGroup III element, and a Group V element; and a shell overcoating thecore and comprising ZnSeS multi-layers comprising Zn, Se, and S, whereineach of the ZnSeS multi-layers of the shell comprises a compound of theformula ZnSe_(y)S_((1-y)), wherein 0≤y≤1, and wherein the ZnSeSmulti-layers have a Se:S concentration ratio gradient, wherein the Se:Sconcentration ratio gradient comprises an increasing to concentration ofSe and a decreasing concentration of S in a direction from the core to apredetermined monolayer; and a decreasing concentration of Se and anincreasing concentration of S in a direction from the predeterminedmonolayer to an outermost monolayer, wherein the predetermined monolayeris located between a first monolayer directly disposed on the surface ofthe core and the outermost monolayer.
 2. The nanocrystal of claim 1,wherein the average ratio of y:(1−y) ranges from about 5:1 to about 20:1in the first monolayer directly disposed on the surface of the core. 3.The nanocrystal of claim 1, wherein the core further comprises a GroupII metal.
 4. The nanocrystal of claim 1, wherein the core comprises acompound selected from GaN, GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN,InP, InAs, and InSb, GaNP, GaNAs, GaNSb, GaPAs, GaPSb, AlNP, AlNAs,AlNSb, AlPAs, AlPSb, InNP, InNAs, InNSb, InPAs, InPSb, GaAlNP, AlGaN,AlGaP, AlGaAs, AlGaSb, InGaN, InGaP, InGaAs, InGaSb, AlInN, AlInP,AlInAs, and AlInSb, GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs,GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs, InAlPSb,or a combination thereof.
 5. The nanocrystal of claim 3, wherein theGroup II metal is selected from Zn, Cd, Hg, Mg, or a combinationthereof.
 6. The nanocrystal of claim 1, wherein the Group III elementcomprises In, and wherein the Group V element comprises P.
 7. Thenanocrystal of claim 1, wherein the outermost monolayer comprises ZnS.8. The nanocrystal of claim 1, wherein the core comprises InZnP.
 9. Thenanocrystal of claim 1, wherein a photoluminescence of the nanocrystalhas a full width at half maximum of less than or equal to about 45nanometers.
 10. The nanocrystal of claim 1, wherein the nanocrystal hasa luminous efficiency QY of greater than or equal to about 70 percent.11. The nanocrystal of claim 1, wherein the nanocrystal has a diameterof greater than or equal to about 6 nanometers.
 12. The nanocrystal ofclaim 1, wherein the nanocrystal has a light emitting region in aphotoluminescence spectrum of about 500 nanometers to about 750nanometers.
 13. The nanocrystal of claim 1, wherein the nanocrystal hasa photoluminescence emission peak at a wavelength ranging from 535nanometers to 545 nanometers.
 14. A method of preparing a nanocrystalcomprising a core comprising a Group III element, and a Group V element,and a shell comprising to ZnSeS multi-layers comprising Zn, Se, and S,and formed on a surface of the core, the method comprising: providing ananocrystal core comprising a Group III element, and a Group V element,and contacting the nanocrystal core with a precursor of Zn, Se, and S toform the ZnSeS multi-layers of the shell overcoating the core andprepare the nanocrystal, wherein each of the ZnSeS multi-layers of theshell comprising a compound of the formula ZnSe_(y)S_((1-y)), wherein0≤y≤1, and wherein forming the ZnSeS multi-layers comprises introducingZn, Se and S precursors in a mole ratio such that the ZnSeS multi-layershave a Se:S concentration ratio gradient, wherein the Se:S concentrationratio gradient comprises an increasing concentration of Se and adecreasing concentration of S in a direction from the core to apredetermined monolayer; and a decreasing concentration of Se and anincreasing concentration of S in a direction from the predeterminedmonolayer to an outermost monolayer, wherein the predetermined monolayeris located between a first monolayer directly disposed on the surface ofthe core and the outermost monolayer.
 15. The method of claim 14,wherein the Zn, Se, and S precursors are present in a mole ratio ofabout 1:2 to about 60:1 to form the first monolayer directly disposed onthe surface of the core to have a Se:S mole ratio of from about 5:1 toabout 20:1.
 16. The method of claim 14, wherein the core furthercomprises a Group II metal.
 17. The method of claim 16, wherein theGroup II metal is selected to from Zn, Cd, Hg, Mg, and a combinationthereof.
 18. The method of claim 14, wherein the outermost monolayercomprises ZnS.
 19. A light emitting device comprising the nanocrystalaccording to claim
 1. 20. The light emitting device of claim 19, whereinthe light emitting device is a display, a sensor, a photodetector, asolar cell, a hybrid composite, or a bio-labeling device.